Third Generation Partnership Project (3GPP) defines the S1 interface for interconnecting the evolved NodeB (eNB, eNodeB) component of the evolved universal terrestrial radio access network (E-UTRAN) to the core network (CN) of the evolved packet system (EPS). The E-UTRAN architecture consists of a set of eNodeBs connected to the evolved packet core (EPC) through the S1 interface. The set of eNodeBs may include a Home eNodeB (HeNB) and a HeNB gateway (HeNB GW). For example, the S1-AP interface exists between the MME and the HeNB GW and the HeNB GW and the HeNB. Thus, the HeNB GW may perform the functionality of an MME or an eNodeB. As used herein, the term “eNodeB” may refer to an eNodeB or a HeNB GW, and the term “MME” may refer to an MME or a HeNB GW. The overall long term evolution (LTE) architecture and E-UTRAN architecture are described in 3GPP TS 36.401.
The S1 interface is specified at the boundary between the EPC and the E-UTRAN. From the S1 perspective, the E-UTRAN access point is an eNodeB, and the EPC access point is either the control plane mobility management entity (MME) logical node or the user plane serving-gateway (S-GW) logical node. The Stream Control Transmission Protocol (SCTP), specified in IETF RFC 4960, is used to support the exchange of S1 Application Protocol (S1-AP) signaling messages between an eNodeB and an MME.
One function of S1-AP is managing UE contexts (i.e., establishing and releasing UE contexts in the eNodeB and in the EPC to support user individual signaling on S1). The S1 UE context management function supports the establishment of the necessary overall initial UE context including E-UTRAN Radio Access Bearer (E-RAB) context, security context, roaming and access restrictions, etc. The establishment of the overall initial UE context is initiated by the MME. The S1 UE context management function also supports the release of the context previously established in the eNodeB. The release of the context may be triggered by the MME either directly or following a request received from the eNodeB.
The eNodeB generally downloads most of its UE context information from the MME. The SCTP protocol provides IP level communication between the MME and eNodeB nodes and it transports the S1-AP messages. SCTP is used for reliable, in sequence delivery of messaging between the two SCTP endpoints for the duration of the link association. According to 3GPP specifications, if the SCTP association is lost, then the eNodeB deletes all its UE contexts. The UEs perform Service Request procedures and download a UE context again from the MME in response to an S1-AP Setup message. This may, for example, result in audio disruption during a Voice over LTE call, or dropped video frames for a video stream.
Specifically, 3GPP TS 36.413 release 12 subclause 8.7.3.1 describes that the purpose of the S1 Setup procedure is to exchange application level data needed for the eNodeB and the MME to correctly interoperate on the S1 interface. Setup is the first S1-AP procedure triggered after the transport network layer (TNL) association becomes operational. It uses non-UE associated signaling. The setup procedure reinitializes the E-UTRAN S1-AP UE contexts (if any) and erases all related signaling connections in the two nodes like a Reset procedure would do, and clears MME overload state information at the eNodeB.
This behavior is appropriate when, for example, the eNodeB, MME, or HeNB GW experiences a failure in which it loses all SCTP and UE context information. In other situations, however, such as when one processor performs the SCTP functions and a different processor stores the UE context information, the behavior is inefficient. For example, a large eNodeB, large HeNB BW, or MME may have 150,000 connected users. Resetting all the connections generates release, setup, and/or modify request/responses for each connection. This could potentially add up to almost a million messages traversing the network as part of an eNodeB reset.
One alternative is to replicate the SCTP state and all non-acknowledged data between two different processors. At high signaling rates, however, such replication requires enormous processing power and a two-phase commit to replicate state data at both boards, which can be prohibitively expensive. Thus, in conventional systems, the SCTP state may be lost on most hardware faults associated with the SCTP process.
As another alternative, some equipment vendors provide proprietary implementations that assume that if an eNodeB does not send an S1 Setup message, then the eNodeB preserved its UE contexts. Such an implementation ignores the 3GPP requirement that the S1 Setup message be sent as the first message in a new SCTP connection. Another problem is that the MME may autonomously delete an SCTP association and related UE contexts because of an SCTP timeout at the MME, which results in stranded UE contexts on the eNodeB. This may also result in non-responsive S1-AP connections to an MME when the MME is waiting for an S1 Setup before providing full service to an eNodeB. Thus, such implementations are unreliable because the circumstances of an SCTP restart/setup may not be clear to each SCTP peer.